I was curious what notes people on this board tune to. On my old shamisen I tuned up to cfc, however on my new one I can’t tune higher than gcg without risking the san no ito snapping from tension. (Needless to say, my first string went quite quickly as I thoughtlessly attempted to tune it to a high c.) It is frustrating having to play halfway down the neck to get to the notes I’m used to hearing however.
Sounds like the instrument might be the problem. Today most players have adopted the outter strings to both be tuned to c. Some players like Masahiro Nitta tune considerably even higher than that. Of course it does depend on the song your playing. I play a lot in b and a personally. But you might want to get your instrument looked at. Anyway hope you get back to playing the the keys you’re used to soon!
Where does the string snap? If it is near the koma it could be a sharp edge in the notch of the koma that you can gently smooth.